FIG. 8 shows one example of a conventional portable brush cutter. This portable brush cutter 1′ shown therein includes, at a distal end of the operating rod 2 thereof, a cutting blade mechanism 3 constituted by a cutting blade 4, a gear case (bevel gear unit) 19 and a safety cover 5. The portable brush cutter 1′ also includes, at a proximal end of the operating rod 2 thereof, a centrifugal clutch-attached internal combustion engine (an air cooling type small two-stroke cycle gasoline engine) 6 equipped with a recoil starter 8 and a fuel tank 9. This engine 6 acts as a prime mover for driving the cutting blade 4 by way of a transmission shaft 7 installed inside the operating rod 2, the shaft 7 being coupled to a bevel gear unit.
Near the center portion of the operating rod 2, a U-shaped handle 10 for swinging the cutting blade mechanism 3 right and left or for moving it up and down is attached. A left grip 11 and a right grip 12 are mounted at left and right ends of this U-shaped handle 10, respectively. The portable brush cutter 1′ constructed in this manner however is accompanied with a problem that, due to the fluctuation of torque, the fluctuation of rotational speed, and the fluctuation of load to be imposed on the cutting blade 4 on the occasion of accelerating or decelerating the engine, torsional vibration occurs at the transmission shaft 7. More specifically, since the diameter of the transmission shaft 7 is small in comparison to its length (for example, 6 mm in diameter and about 1500 mm in length), when the rotational speed of engine or the load to be imposed on the cutting blade 4 is increased, the transmission shaft 7 gets distorted (rotational delay). Further, when the load acting on the cutting blade 4 is suddenly reduced, a phenomenon of spring back occurs in the transmission shaft 7, whereby the transmission shaft 7 is permitted to overrun in the rotating direction thereof and twisted. Due to the fluctuation of torque, the fluctuation of rotational speed, the fluctuation of load on the cutting blade, and the spring back phenomenon on the occasion of accelerating or decelerating the engine as described above, the transmission shaft 7 twists or reverts in the rotating direction or in the backward direction, thus generating the torsional vibration.
This torsional vibration of the transmission shaft 7 is then transmitted, through the operating rod 2 and the handle 10, to the hands of a working operator. This kind of vibration may not only lead to the deterioration of operating performance of the cutter and to the increase of fatigue but also may become a cause for a white finger disease especially when the operation of the cutter is continued for long hours. Therefore, to take measures for suppressing this torsional vibration is now an important issue in the manufacture of this portable brush cutter.
JP Utility Model Publication No. 56-53618 (1981) proposes one of the measures to suppress the torsional vibration, wherein a coupler formed of vibration proof rubber is interposed between the centrifugal clutch and the transmission shaft.
JP Patent Laid-open Publication (Kokai) No. 2003-88220 (2003) proposes another measures to suppress the torsional vibration, wherein a one-way clutch is interposed between the centrifugal clutch and the transmission shaft.
There has been also studied, as alternative one of the measures to suppress the torsional vibration, to interpose a torsion coil spring between the centrifugal clutch and the transmission shaft.
However, these conventional measures to use a coupler formed of vibration proof rubber, a one-way clutch or a torsion coil spring for suppressing the torsional vibration are not fully effective, even though some degree of effects to suppress the torsional vibration can be obtained.
Specifically, when a coupler formed of vibration proof rubber is used, the coupler generally has poor durability. Further, even when a torsion coil spring is used, the torsion coil spring may be deformed beyond its elastic limits depending on the twisting direction or the magnitude of twist (angle of torsion) of the transmission shaft. This may result in the torsion coil spring becoming easily fractured and hence deteriorating the durability thereof.
The present invention has been made with a view to overcome the problems of prior art described above, and therefore an object of the present invention is to provide a vibration-absorbing coupler which is capable of effectively suppressing the torsional vibration of transmission shaft and of enhancing the durability thereof. Another object of the present invention is to provide a portable brush cutter equipped with such a vibration-absorbing coupler.